Well, the typical solution to prevent entering data twice in the database is to build an index on one or more of its columns, and mark the index as unique. This prevents duplicate data being entered. Considering you don't have an index of some sorts, you'll need to scan your file each time, to see whether the information has been entered already.

Or perhaps you mean that you don't want the same user to enter information twice. In that case, use some form of authentication, and keep track who has submitted data, and who hasn't. Check before storing the data. Or index your data keyed on the user, and simply replace the data with the new one.